1. Field of the Invention
This invention relates to a new process for development in electrophotographic processes where insulating magnetic toners are employed.
2. Description of the Prior Art
There are known many electrophotographic processes such as those disclosed in U.S. Pat. No. 2,297,691, British Pat. Nos. 1,165,406 and 1,164,405. Most widely used are processes comprising utilizing a photoconductive material, forming electric latent image on a photosensitive material by an optional means, developing the latent images with a toner, if desired, transferring the images thus developed to an image receiving member such as paper, and then fixing the developed toner images by heat, pressure, solvent vapor or the like. There are known various methods for visualizing electric latent images by a toner. For example, there are known magnet brush methods as disclosed in U.S. Pat. No. 2,874,063, cascade developing methods as disclosed in U.S. Pat. No. 2,618,552, powder cloud methods as disclosed in U.S. Pat. No. 2,221,776, fur brush methods, liquid developing methods and the like.
Among these developing methods, there are widely used, in practice, magnet brush methods, cascade methods, liquid developing methods and the like where the developer is mainly composed of toner and a carrier. These developing methods can produce relatively stably a good image, but suffer from degradation of carrier and variation of the mixing ratio of the toner and the carrier which are common and inherent drawbacks of two-component developers.
For the purpose of avoiding such drawbacks, it has been proposed to use one-component developers composed of toner only, and among them, methods using a developer composed of magnetic toner particles give a good result.
U.S. Pat. No. 3,909,258 disclosed a process for developing with a magnetically attractable, electronically conductive toner where a developer composed of the toner is carried on a conductive sleeve of drum type having magnets inside and the development is carried out by contacting the developer with electrostatic images. At the developing portion an electrically conductive path is formed by the toner paticles between the surface of an image receiving member and the sleeve surface, and electric charge is led from the sleeve to the toner particles through the electronically conductive path, and the toner particles attach to the image portions by Coulomb force to develop the image portions.
The above mentioned developing method using the magnetically attractable, electronically conductive toner is a good method free from inherent problems of two-component developing methods, but it is difficult to transfer electrostatically the developed images to a final support such as plain paper from the developed image bearing member because the toner is electrically conductive.
As a developing method where a highly resistive and magnetic toner capable of being transferred electrostatically is employed, Japanese Patent Laid-Open No. 94140/1977 discloses a process for development utilizing induction polarization of toner particles, but the process suffers from disadvantages such as low developing speed and insufficient density of the developed images, and is practised with difficulty.
A further method of development using a highly resistive and magnetic toner is a method comprising triboelectorically charging the toner particles by the friction of toner particles contacting each other, the friction between toner particles and a sleeve, and the like, and bringing the toner particles thus charged into contact with an electrostatic image bearing member to develop the electrostatic images. However, this method suffers from the disadvantages tht the amount of contact between the toner paticles and the friction member is too small to be sufficiently charged and the toner particles thus charged are more strongly affected by Coulomb force between the toner particles and the sleeve and thereby are liable to agglomerate on the sleeve. The practical operation is effected with difficulty.
One-component developing methods practically used up to now are the method of U.S. Pat. No. 3,909,258 and methods similar thereto. The practically important features of this U.S. Patent are that the toner is conductive and the toner contains 50% by weight or more of a magnetic material. These two features are adventageous in the developing step, but the former is disadvantageous in the transferring step and the latter is disadvantageous in the fixing step. In other words, since the toner is conductive, and electrostatic transferring is difficult as mentioned above and even if the resistance is adequately controlled, the transferring efficiency is remarkably lowered. In addition, the content of magentic materials in the toner is as high as 50% by weight or more so that a lot of energy is necessary for fixing by heated press rollers. In other words, when a large amount of non-melting materials is contained in the toner, contact between toner particles one another or between toner particles and a toner image bearing member is not so tight that a lot of heat energy or a high pressure is required.